Stacked gate-all-around FinFET and method forming the same

ABSTRACT

A device includes a first semiconductor strip, a first gate dielectric encircling the first semiconductor strip, a second semiconductor strip overlapping the first semiconductor strip, and a second gate dielectric encircling the second semiconductor strip. The first gate dielectric contacts the first gate dielectric. A gate electrode has a portion over the second semiconductor strip, and additional portions on opposite sides of the first and the second semiconductor strips and the first and the second gate dielectrics.

PRIORITY CLAIM

This application is a divisional of U.S. patent application Ser. No.14/675,160, filed on Mar. 31, 2015, entitled “Stacked Gate-All-AroundFinFET and Method Forming the Same,” which claims priority to U.S.Provisional Application No. 62/115,558, filed Feb. 12, 2015, entitled,“Stacked Gate-All-Around FinFET and Method Forming the Same,” both ofwhich applications are hereby incorporated herein by reference as ifreproduced in their entireties.

CROSS-REFERENCE

This application relates to the following commonly-assigned U.S. patentapplication: application Ser. No. 14/317,069, filed Jun. 27, 2014, andentitled “Method of Forming Semiconductor structure with Horizontal GateAll Around Structure,” which application is hereby incorporated hereinby reference.

BACKGROUND

Technological advances in Integrated Circuit (IC) materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generations. In the course of ICevolution, functional density (for example, the number of interconnecteddevices per chip area) has generally increased while geometry sizes havedecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,Fin Field-Effect Transistors (FinFETs) have been introduced to replaceplanar transistors. The structures of FinFETs and methods of fabricatingFinFETs are being developed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 21D are cross-sectional views and perspective views ofintermediate stages in the formation of a Fin Field-Effect Transistor(FinFET) in accordance with some exemplary embodiments;

FIG. 22 illustrates a process flow for forming a FinFET in accordancewith some embodiments;

FIGS. 23A, 23B, and 23C illustrate the cross-sectional views of channelregions and gate stacks of FinFETs in accordance with some embodiments;

FIGS. 24 through 40C illustrate cross-sectional views, top views, andperspective views in the formation of a FinFET in accordance with someexemplary embodiments; and

FIG. 41 illustrates a process flow for forming a FinFET in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Fin Field-Effect Transistors (FinFETs) with Gate-All-Around (GAA)structures and the methods of forming the same are provided inaccordance with various exemplary embodiments. The intermediate stagesof forming the FinFETs are illustrated. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.It is appreciated that although FIGS. 1 through 23C and FIGS. 24 through40C illustrate different embodiments, these embodiments may be combinedin the formation of the same FinFET. For example, the embodiments shownin FIGS. 1 through 23C include the formation of channel regions and gatestacks of FinFETs, and the embodiments shown in FIGS. 24 through 40Cinclude the formation of the source/drain regions and source/drainsilicides of FinFETs. The formation of the channel regions and gatestacks and the formation of the source/drain regions and source/drainsilicides in accordance with the embodiments of the present disclosuremay thus be combined to form a FinFET.

FIGS. 1 through 21D illustrate the perspective views and cross-sectionalviews of intermediate stages in the formation of a FinFET in accordancewith some embodiments. The steps shown in FIGS. 1 through 21D are alsoillustrated schematically in the process flow 300 shown in FIG. 22. Inthe subsequent discussion, the process steps shown in FIGS. 1 through21D are discussed referring to the process steps in FIG. 22.

FIG. 1 illustrates a cross-sectional view of substrate 20, which may bea part of a wafer. Substrate 20 may be a semiconductor substrate, whichmay further be a silicon substrate, a silicon carbon substrate, asilicon-on-insulator substrate or a substrate formed of othersemiconductor materials. Substrate 20 may be lightly doped with a p-typeor an n-type impurity. An Anti-Punch-Through (APT) implantation(illustrated by arrows) is then preformed on a top portion of substrate20 to form APT region 21. The respective step is shown as step 302 inthe process flow shown in FIG. 22. The conductivity type of the dopantsimplanted in the APT is the same as that of the well region (not shown).APT layer 21 extends under the subsequently formed source/drain regions58 (FIG. 21A), and are used to reduce the leakage from source/drainregions 58 to substrate 20. The doping concentration in APT layer 21 maybe in the range between about 1E18/cm³ and about 1E19/cm³. For clarity,in subsequent drawings, APT region 21 is not illustrated.

Referring to FIG. 2, silicon germanium (SiGe) layer 22 and semiconductorstack 24 are formed over substrate 20 through epitaxy. The respectivestep is shown as step 304 in the process flow shown in FIG. 22.Accordingly, SiGe layer 22 and semiconductor stack 24 form crystallinelayers. In accordance with some embodiments of the present disclosure,the thickness T1 of SiGe layer 22 is in the range between about 5 nm andabout 8 nm. The germanium percentage (atomic percentage) of SiGe layer22 is in the range between about 25 percent and about 35 percent, whilehigher or lower germanium percentages may be used. It is appreciated,however, that the values recited throughout the description areexamples, and may be changed to different values.

Over SiGe layer 22 is semiconductor stack 24. In accordance with someembodiments, semiconductor stack 24 comprises semiconductor layers 26and 28 stacked alternatively. Semiconductor layers 26 may be puresilicon layers that are free from germanium. Semiconductor layers 26 mayalso be substantially pure silicon layers, for example, with a germaniumpercentage lower than about 1 percent. Furthermore, semiconductor layers26 may be intrinsic, which are not doped with p-type and n-typeimpurities. There may be two, three, four, or more of semiconductorlayers 26. In accordance with some embodiments, thickness T2 ofsemiconductor layers 26 is in the range between about 6 nm and about 12nm.

Semiconductor layers 28 are SiGe layers having a germanium percentagelower than the germanium percentage in SiGe layer 22. In accordance withsome embodiments of the present disclosure, the germanium percentage ofSiGe layers 28 is in the range between about 10 percent and about 20percent. Furthermore, a difference between the germanium percentage ofSiGe layer 22 and the germanium percentage of SiGe layers 28 may begreater than about 15 percent or higher. In accordance with someembodiments, thickness T3 of SiGe layers 28 is in the range betweenabout 2 nm and about 6 nm.

Hard mask 30 is formed over semiconductor stack 24. In accordance withsome embodiments of the present disclosure, hard mask 30 is formed ofsilicon nitride, silicon oxynitride, silicon carbide, siliconcarbo-nitride, or the like.

Next, as shown in FIG. 3, hard mask 30, semiconductor stack 24, SiGelayer 22 and substrate 20 are patterned to form trenches 32. Therespective step is shown as step 306 in the process flow shown in FIG.22. Accordingly, semiconductor strips 34 are formed. Trenches 32 extendinto substrate 20, and have lengthwise directions parallel to eachother. The remaining portions of semiconductor stack 24 are accordinglyreferred to as semiconductor strips 24 alternatively.

Referring to FIG. 4, an oxidation process is performed on the exposedportions of semiconductor strips 34. The respective step is shown asstep 308 in the process flow shown in FIG. 22. In accordance with someembodiments of the present disclosure, before the oxidation, a trim stepis performed to trim SiGe strips 22 and 28, with silicon strips 26 nottrimmed. The trimming results in SiGe layers 22 and 28 to be recessedlaterally from the respective edges of silicon strips 26. The trimminghas the effect of reducing the width of SiGe layer 22, so that in thesubsequent oxidation, SiGe layer 22 can be fully oxidized withoutrequiring the time and/or temperature for the oxidation to be increasedtoo much.

As a result of the oxidation, SiGe layer 22 is fully oxidized to formsilicon germanium oxide regions 38, and at least the outer portions ofSiGe strips 28 are oxidized to form silicon germanium oxide regions 40.The thickness of silicon germanium oxide regions 38 may be in the rangebetween about 5 nm and about 20 nm. In some embodiments, the oxidationis performed at a temperature in the range between about 400° C. and600° C. The oxidation time may range between about 2 minutes and about 4hours, for example. The oxidation of silicon in silicon germanium iseasier than the oxidation of germanium in the same silicon germaniumregion. Accordingly, the silicon atoms in semiconductor strips 28 areoxidized, and the germanium atoms in semiconductor strips 28 may diffuseinwardly toward the centers of SiGe strips 28, and hence the germaniumpercentage in the remaining SiGe strips 28 is increased over that in theSiGe strips 28 before the oxidation.

During the oxidation, silicon oxide layers 36 are also formed on theexposed surfaces of substrate 20 and silicon strips 26. Since theoxidation rate of SiGe (or silicon) regions increase with the increasein the percentages of germanium, the oxidation of silicon layers 26 andsubstrate 20 is much slower than the oxidation of SiGe layer 22 and SiGestrips 28. Accordingly, silicon oxide layers 36 are thin, and themajority of silicon layers 26 and the portions (referred to as stripportions hereinafter) of substrate 20 in strips 34 are not oxidized.

Next, as shown in FIG. 5, isolation regions 42, which may be ShallowTrench Isolation (STI) regions, are formed in trenches 32 (FIG. 4). Theformation may include filling trenches 32 with a dielectric layer(s),for example, using Flowable Chemical Vapor Deposition (FCVD), andperforming a Chemical Mechanical Polish (CMP) to level the top surfaceof the dielectric material with the top surface of hard mask 30. Afterthe CMP, hard mask layer 30 (FIG. 4) is removed.

Next, referring to FIG. 6, STI regions 42 are recessed. The steps shownin FIGS. 5 and 6 are shown as step 310 in the process flow shown in FIG.22. The top surface 42A of the resulting STI regions 42 may be levelwith the top surface or the bottom surface of silicon germanium oxideregion 38, or may be at any intermediate level between the top surfaceand the bottom surface of silicon germanium oxide regions 38. Throughoutthe description, semiconductor stack 24 is also referred to assemiconductor fins 24 hereinafter.

FIG. 7 illustrates the formation of dummy oxide layer 44, which mayinclude silicon oxide in accordance with some embodiments. Hence, dummyoxide layer 44 protects the sidewalls of semiconductor strips 24,silicon germanium oxide regions 38, and the top surfaces ofsemiconductor stack 24. Dummy oxide layer 44 also extends on the topsurfaces of STI regions 42. Since dummy oxide layer 44 and STI regions42 may be formed of the same dielectric material (such as siliconoxide), the interface between dummy oxide layer 44 and STI regions 42are not shown although they are distinguishable in some embodiments. Inother embodiments, the interface is not distinguishable.

Referring to FIG. 8, dummy gate stack 46 is formed. The respective stepis shown as step 312 in the process flow shown in FIG. 22. In accordancewith some embodiments of the present disclosure, dummy gate stack 46includes dummy gate electrode 48, which may be formed, for example,using polysilicon. Dummy gate stack 46 may also include hard mask layer50 over dummy gate electrode 48. Hard mask layer 50 may include siliconnitride and/or silicon oxide, for example, and may be a single layer ora composite layer including a plurality of layers. In some embodiments,hard mask layer 50 includes silicon nitride layer 50A and silicon oxidelayer 50B over silicon nitride layer 50A. Dummy gate stack 46 has alengthwise direction substantially perpendicular to the lengthwisedirection of semiconductor fins 24.

Gate spacers 54 are formed on the sidewalls of dummy gate stack 46. Inaccordance with some embodiments of the present disclosure, gate spacers54 are formed of silicon nitride, and may have a single-layer structure.In alternative embodiments, gate spacers 54 have a composite structureincluding a plurality of layers. For example, gate spacers 54 mayinclude a silicon oxide layer, and a silicon nitride layer over thesilicon oxide layer. Dummy gate stack 46 and gate spacers 54 cover amiddle portion of each of semiconductor fins 24, leaving the oppositeend portions not covered.

FIG. 9 illustrates the removal of the end portions of semiconductor fins24. A dry etch(es) is performed to etch dummy oxide layer 44,semiconductor stacks 24, and silicon germanium oxide regions 38 as shownin FIG. 8. As a result, recesses 56 are formed. In accordance with someembodiments of the present disclosure, some portions of dummy oxidelayer 44 may be left standing over and aligned to the edges of STIregions 42, with recesses 56 formed therebetween.

Next, referring to FIG. 10, epitaxy regions (source/drain regions) 58are formed by selectively growing a semiconductor material from recesses56 (FIG. 9). In accordance with some embodiments of the presentdisclosure, the formation of source/drain regions 58 includes an epitaxygrowth. In accordance with alternative embodiments, the formation ofsource/drain regions 58 is achieved by adopting the process steps shownin FIGS. 28 through 37A. The respective step is shown as step 314 in theprocess flow shown in FIG. 22. As illustrated in FIG. 10, due to theblocking of the remaining portions of dummy oxide layer 44, source/drainregions 58 are first grown vertically in recesses 56 (FIG. 9), duringwhich time source/drain regions 58 do not grow horizontally. Afterrecesses 56 are fully filled, source/drain regions 58 are grown bothvertically and horizontally to form facets.

In some exemplary embodiments in which the resulting FinFET is an n-typeFinFET, source/drain regions 58 comprise silicon phosphorous (SiP) orphosphorous-doped silicon carbon (SiCP). In alternative exemplaryembodiments in which the resulting FinFET is a p-type FinFET,source/drain regions 58 comprise SiGe, and a p-type impurity such asboron or indium may be in-situ doped during the epitaxy.

Next, as shown in FIG. 11, Inter-Layer Dielectric (ILD) 60 is formed.The respective step is shown as step 316 in the process flow shown inFIG. 22. A CMP is then performed to level the top surfaces of ILD 60,dummy gate stack 46 (FIG. 10), and gate spacers 54 (FIG. 10) with eachother. Each of ILD 60, gate spacers 54, and insulation regions 42 mayhave distinguishable interfaces with the other ones of ILD 60, gatespacers 54, and STI regions 42 since they are formed in differentprocess steps, having different densities, and/or comprise differentdielectric materials.

Next, dummy gate 46 as shown in FIG. 10 is removed in an etching step,so that recess 62 is formed to extend into ILD 60, as shown in FIG. 11.The respective step is shown as step 318 in the process flow shown inFIG. 22. To illustrate the features behind the front portion of ILD 60,some front portions of ILD 60 are not shown in subsequent figures, sothat the inner features may be illustrated. It is appreciated that theun-illustrated portions of ILD 60 still exist. After the removal ofdummy gate stack 46, the middle portions of semiconductor stacks (fins)24 are exposed to recess 62. During the removal of dummy gate stack 46,dummy gate oxide 44 (FIG. 7) is used as an etch stop layer when the toplayer is etched. Dummy gate oxide 44 is then removed, and hencesemiconductor fins 24 are exposed to recess 62.

Referring to FIG. 12A, an etching step is performed to remove silicongermanium oxide regions 40 (also refer to FIG. 9), concentratedsemiconductor strips 28, and some top portions of silicon germaniumoxide regions 38 (FIG. 8). The respective step is shown as step 320 inthe process flow shown in FIG. 22. Accordingly, silicon strips 26 areseparated from each other by gaps 64. In addition, the bottom one ofsilicon strips 26 may also be separated from the remaining silicongermanium oxide regions 38 by gaps 64. As a result, silicon strips 26are suspended. The opposite ends of the suspended silicon strips 26 areconnected to source/drain regions 58. It is appreciated that STI regions42 include first portions underlying and exposed to recess 62, andsecond portions covered by gate spacers 54 and ILD 60. In accordancewith some embodiments of the present disclosure, the top surfaces of thefirst portions of STI regions 42 are recessed to be lower than the topsurfaces of the second portions of STI regions 42.

FIG. 12B illustrates a clearer view of the portions of silicon strips26. ILD 60, source/drain regions 58, and gate spacers 54 as shown inFIG. 12A are not shown in FIG. 12B, although these features still exist.

Referring to FIGS. 13A and 13B, an oxidation step is performed. Therespective step is shown as step 322 in the process flow shown in FIG.22. FIG. 13B also illustrates some portions of the structure shown inFIG. 13A, wherein ILD 60, source/drain regions 58, and gate spacers 54as shown in FIG. 13A are not shown in FIG. 13B, although these featuresstill exist. The oxidation may be performed using steam oxidation inwater steam, thermal oxidation in oxygen (O₂), or the like. Inaccordance with some embodiments of the present disclosure, theoxidation is performed using water steam at a temperature in the rangebetween about 400° C. and about 600° C. The duration of the oxidationmay be in the range between about 20 seconds and about 20 minutes. As aresult of the oxidation, the outer portions of silicon strips 26 areoxidized to form silicon oxide rings 66, which encircle the remainingportions of silicon strips 26, as shown in FIG. 13B. In someembodiments, the silicon oxide rings 66 formed from neighboring siliconstrips 26 touch each other. In addition, the silicon oxide rings 66formed from the bottom one of silicon strips 26 may be in contact withthe top surface of silicon germanium oxide regions 38 in someembodiments. In alternative embodiments, the silicon oxide rings 66formed from neighboring silicon strips 26 are spaced apart from eachother.

In the oxidation, the core FinFETs in core circuits (also known as logiccircuits) and the input/output (IO) FinFETs in IO circuits have theirsemiconductor strips oxidized simultaneously. The structure shown inFIGS. 13A and 13B illustrate the structures of both the core FinFETs andthe IO FinFETs. In a subsequent step, as shown in FIG. 14A, photo resist68 is formed to cover silicon oxide rings 66 in IO region 200, whilecore region 100 is not covered. An etching is then performed to removesilicon oxide rings 66 in core region 100, so that silicon strips 26 areexposed. On the other hand, the silicon oxide rings 66 in IO region 200are protected from the removal, and hence will remain after the etching.The respective step is shown as step 324 in the process flow shown inFIG. 22. Photo resist 68 is then removed. After the etching, siliconstrips 26 in core region 100 are again separated from each other bygaps, and the bottom one of silicon strips 26 is separated from the topsurface of silicon germanium oxide region 38 by a gap. FIG. 14Billustrates some portions of the structure shown in FIG. 14A, whereinILD 60, source/drain regions 58, and gate spacers 54 as shown in FIG.14A are not shown in FIG. 14B, although these features still exist.

In a subsequent step, gate dielectric 70 is formed. The respective stepis shown as step 326 in the process flow shown in FIG. 22. For a coreFinFET in the core region (100 in FIGS. 14A and 14B), gate dielectric 70is formed on the exposed surfaces of silicon strips 26 (FIGS. 14A and14B). The resulting structure is shown in FIGS. 15A and 15B. For an IOFinFET in the IO region (200 in FIGS. 14A and 14B), gate dielectric 70is formed on the already formed silicon oxide rings 66, and hencesilicon oxide rings 66 become parts of gate dielectric 70. Accordingly,both the core FinFET and the IO FinFET have the structure shown in FIGS.15A and 15B, except that the gate dielectric 70 of an IO FinFET isthicker than that of a core FinFET. Again, FIG. 15B also illustratessome portions of the structure shown in FIG. 15A, wherein ILD 60,source/drain regions 58, and gate spacers 54 as shown in FIG. 15A arenot shown in FIG. 15B.

In accordance with some embodiments, the formation of gate dielectric 70includes performing an interfacial (dielectric) layer, and then forminga high-k dielectric layer on the interfacial layer. The interfaciallayer may include silicon oxide formed by treating the structure inFIGS. 14A and 14B in a chemical solution, so that silicon strips 26 areoxidized to form a chemical oxide (silicon oxide). As a result, the gatedielectric 70 in the IO region will be thicker than the gate dielectric70 in the core region. The high-k dielectric is then deposited on theinterfacial layer. In some embodiments, the high-k dielectric has a kvalue greater than about 7.0, and may include a metal oxide or asilicate of Hf, Al, Zr, La, and the like.

FIG. 16A illustrates the formation of gate electrode 72. The respectivestep is shown as step 328 in the process flow shown in FIG. 22. Theformation includes filling recess 62 (FIG. 15A) with a conductivematerial, and performing a planarization such as a CMP. Gate electrode72 may include a metal-containing material such as TiN, TaN, TaC, Co,Ru, Al, Cu, W, combinations thereof, or multi-layers thereof. FinFET 74is thus formed. Anti-punch-through region 21 is underlying silicongermanium oxide region 38 and source/drain regions 58.

FIGS. 16B and 16C illustrate the cross-sectional views of some portionsof FinFET 74 in FIG. 16A, wherein the cross-sectional views are obtainedfrom the vertical plane containing line 16B/16C-16B/16C in FIG. 16A. Asshown in FIGS. 16B and 16C, gate dielectric 70 fully fills the gapbetween neighboring silicon strips 26. Accordingly, gate electrode 72will not be able to be filled into the gap between neighboring siliconstrips 26, and will not be shorted to source/drain regions 58 (FIG.16A).

FIGS. 16B and 16C also illustrate that gate dielectric 70 includessilicon oxide 76 and high-k dielectric 78 on the outer side of siliconoxide 76. When FinFET 74 is a core FinFET, silicon oxide 76 includes theinterfacial layer. When FinFET 74 is an IO FinFET, silicon oxide 76includes silicon oxide rings 66 (FIGS. 13A and 13B) and an interfaciallayer. In FIG. 16B, silicon oxide 76 formed on neighboring siliconstrips 26 contact with each other in accordance with some embodiments.In FIG. 16C, silicon oxides 76 formed on neighboring silicon strips 26do not contact with each other, and high-k dielectric 78 fills the gapbetween the silicon oxides 76 that are formed on neighboring siliconstrips 26 in accordance with some embodiments.

FIGS. 17A through 22B illustrate cross-sectional views of intermediatestages in the formation of a FinFET in accordance with alternativeembodiments. Unless specified otherwise, the materials and the formationmethods of the components in these embodiments are essentially the sameas the like components, which are denoted by like reference numerals inthe embodiments shown in FIGS. 1 through 16C. The details regarding theformation process and the materials of the components shown in FIGS. 17Athrough 22B may thus be found in the discussion of the embodiment shownin FIGS. 1 through 16C.

The initial steps of these embodiments are essentially the same as shownin FIGS. 1 through 11. Next, FIGS. 17A and 17B illustrate the etchingstep similar to the step shown in FIGS. 12A and 12B. Referring to FIG.17A, an etching is performed to remove silicon germanium oxide regions40 (also refer to FIG. 9), concentrated semiconductor strips 28, andsome top portions of silicon germanium oxide region 38 (FIG. 8).Accordingly, silicon strips 26 are separated from each other by gaps 64.In addition, the bottom one of silicon strips 26 may also be separatedfrom the remaining silicon germanium oxide regions 38 by gaps 64.Compared to the step shown in FIGS. 12A and 12B, the portions of STIregions 42 and silicon germanium oxide regions 38 are recessed lowerthan in FIGS. 12A and 12B. As a result, the gaps 64 between the bottomone of silicon strips 26 and the top surface of silicon germanium oxideregions 38 are higher than in FIGS. 12A and 12B.

Next, FIGS. 18A and 18B illustrate essentially the same process step andthe structure as shown in FIGS. 13A and 13B, respectively, wherein anoxidation is performed, and silicon oxide rings 66 are formed. Thebottom ones of silicon oxide rings 66 may be spaced apart from the topsurfaces of silicon germanium oxide regions 38 by gaps 64. FIGS. 19A and19B illustrate essentially the same process step and the structure asshown in FIGS. 14A and 14B, respectively, wherein silicon oxide rings 66are removed from the core device region. In the meanwhile, the siliconoxide rings 66 in the IO region (not shown) are protected, and are notremoved. FIGS. 20A and 20B illustrate essentially the same process stepand the structure as shown in FIGS. 15A and 15B, respectively, whereingate dielectric 70 is formed. FIGS. 21A and 21B illustrate essentiallythe same process step and the structure as shown in FIG. 16A, whereingate electrode 72 is formed.

FIGS. 21C and 21D illustrate the cross-sectional views of some portionsof FinFET 74 in FIG. 21A, wherein the cross-sectional views are obtainedfrom the vertical plane containing line 21C/21D-21C/21D in FIG. 21A. Asshown in FIGS. 21C and 21D, gate dielectric 70 fully fills the gapbetween neighboring silicon strips 26. Accordingly, gate electrode 72 isnot filled into the gaps between neighboring silicon strips 26, and willnot be shorted to source/drain regions 58 (FIG. 16A).

As a result of the deeper recessing of STI regions 42 and silicongermanium oxide region 38 as shown in FIGS. 17A and 17B, STI regions 42and silicon germanium oxide regions 38 are spaced farther away from theoverlying silicon strips 26. As a result, as shown in FIGS. 21C and 21D,at least some top surfaces of silicon germanium oxide region 38 arespaced apart from the gate dielectric 70 that is formed on the bottomones of silicon strips 26. In FIG. 21C, the central portion of silicongermanium oxide region 38 is recessed less, and protrudes over theportions of silicon germanium oxide region 38 on the opposite sides ofthe central portion. Gate dielectric 70 fills the space between thecentral portion of silicon germanium oxide region 38 and the bottomsilicon strip 26. In FIG. 21D, The gate dielectric formed on the bottomsilicon strip 26 is separated from a dielectric (also marked as 70)formed on the top surfaces of silicon germanium oxide region 38 and STIregions 42 by a gap, with gate electrode 72 filling the gap.

FIGS. 23A, 23B, and 23C illustrate the cross-sectional views of channelsand gates of FinFETs in accordance with alternative embodiments. Inthese embodiments, there are two, instead of three or four siliconstrips 26. Furthermore, semiconductor strips 26 may have heights greaterthan the respective widths. For example, the height H1 of each ofsilicon strips 26 may be in the range between about 10 nm and about 30nm, and the widths W1 of each of silicon strips 26 may be in the rangebetween about 6 nm and about 12 nm. FIGS. 23A, 23B, and 23C illustratethe embodiments corresponding to the embodiments shown in FIGS. 16B/16C,21C, and 21D, respectively, and hence the details are not repeatedherein.

The embodiments of the present disclosure have some advantageousfeatures. The anti-punch-through implantation is performed before theformation of the channel material (silicon strips 26). Accordingly, thechannels of the resulting FinFET are not affected by the implanteddopant, and hence the impurity scattering and reduction in carriermobility suffered from the conventional anti-punch-through implantationis eliminated. The resulting FinFET is a GAA FinFET with a plurality ofchannels. Accordingly, the short channel effect related to Drain-InducedBarrier Lowering (DIBL) is improved, and the drive current of the FinFETis improved due to the multiple channels.

FIGS. 24 through 40C illustrate cross-sectional views of intermediatestages in the formation of a FinFET in accordance with alternativeembodiments. Unless specified otherwise, the materials and the formationmethods of the components in these embodiments are essentially the sameas the like components, which are denoted by like reference numerals inthe embodiments shown in FIGS. 1 through 23C. The details regarding theformation process and the materials of the components shown in FIGS. 24through 40C may thus be found in the discussion of the embodiments shownin FIGS. 1 through 23C. The steps shown in FIGS. 24 through 40C are alsoillustrated schematically in the process flow 400 shown in FIG. 41.

FIG. 24 illustrates the formation of an APT implantation (illustrated byarrows) to form anti-punch-through region 21 in semiconductor substrate20. The respective step is shown as step 402 in the process flow shownin FIG. 41. The process step and the process detail is essentially thesame as shown in FIG. 1, and hence are not repeated herein.

Next, as shown in FIG. 25, SiGe layer 22 and semiconductor layer(s) 124are formed over substrate 20 through epitaxy. The respective step isshown as step 404 in the process flow shown in FIG. 41. Accordingly,SiGe layer 22 forms a crystalline layer. The germanium percentage(atomic percentage) of SiGe layer 22 is between about 25 percent andabout 35 percent, while higher or lower germanium percentages may beused. In accordance with some embodiments of the present disclosure,thickness T4 of SiGe layer 22 is in the range between about 5 nm andabout 8 nm.

Semiconductor layer 124 is formed over SiGe layer 22. In accordance withsome embodiments of the present application, semiconductor layer 124 isa single layer formed of a homogenous semiconductor material. Forexample, semiconductor layer 124 may be formed of silicon free fromgermanium therein. Semiconductor layer 124 may also be a substantiallypure silicon layer, for example, with a germanium percentage lower thanabout 1 percent. Furthermore, semiconductor layer 124 may be intrinsic,which is not doped with p-type and n-type impurities. In accordance withsome embodiments, thickness T4 of semiconductor layer 124 is in therange between about 30 nm and about 80 nm.

In accordance with alternative embodiments of the present disclosure,semiconductor layer 124 is a composite layer that is a semiconductorstack having essentially the same structure as semiconductor stack 24 asshown in FIG. 2. Accordingly, the structure and the materials of thecomposite semiconductor layer 124 may be found in the description ofsemiconductor stack 24.

In addition, a hard mask (not shown) may be formed over semiconductorlayer 124. In accordance with some embodiments, the hard mask is formedof silicon nitride, silicon oxynitride, silicon carbide, siliconcarbo-nitride, or the like.

Next, as shown in FIG. 26, the hard mask, semiconductor layer 124, SiGelayer 22 and substrate 20 are patterned to form trenches 32. Therespective step is shown as step 406 in the process flow shown in FIG.41. Accordingly, semiconductor strips 34 are formed. Trenches 32 extendsinto substrate 20, and trenches 32 and semiconductor strips 34 havelengthwise directions parallel to each other. The remaining portions ofsemiconductor layer 124 are accordingly referred to as stripsalternatively. In a subsequent step, trenches 32 are filled with STIregions 42, followed by the recessing of STI regions 42. In FIG. 26 andsubsequent figures, the lower portions of STI regions 42 and substrate20 are not shown. The portions of the structure below semiconductorlayer 22 are essentially the same as the lower parts of the structureshown in FIG. 6, wherein portions of substrate 20 (referred to as asubstrate strip hereinafter) are located between opposite portions ofSTI regions 42.

After the recessing of STI regions 42, the top surfaces of STI regions42 are lower than the top surfaces of SiGe strips 22. In accordance withsome embodiments of the present disclosure, the top surfaces of STIregions 42 are level with or slightly lower than the top surfaces ofSiGe strips 22, so that at least some portions, and possibly theentirety, of the sidewalls of SiGe strips 22 are exposed.

Next, referring to FIG. 27, an oxidation process is performed on theexposed portions of semiconductor strips (fins) 34 to form silicongermanium oxide regions 38. The respective step is shown as step 408 inthe process flow shown in FIG. 41. As a result of the oxidation, SiGelayers 22 are fully oxidized to form silicon germanium oxide regions 38.In accordance with some embodiments, the oxidation is performed at atemperature in the range between about 400° C. and 600° C. The oxidationtime may range between about 2 minutes and about 4 hours, for example.During the oxidation, silicon oxide (not shown) is also formed on theexposed surfaces of semiconductor strips 124. Due to the much loweroxidation rate of silicon than silicon germanium, the silicon oxidelayer on semiconductor strips 124 is thin, and hence is not illustratedherein.

In the embodiments in which semiconductor strips 124 have the samestructure as semiconductor strips 24 as shown in FIG. 3, the resultingstructure after the oxidation will include silicon germanium oxideregions 40, concentrated silicon germanium regions 28, similar to whatis shown in FIG. 4.

Next, as shown in FIGS. 28 through 30, etch stop layer 122 is formed.The respective step is shown as step 410 in the process flow shown inFIG. 41. Etch stop layer 122 acts as an etch stop layer in thesubsequent formation of contact opening for forming source/drainsilicides and source/drain contacts. In accordance with some embodimentsof the present disclosure, etch stop layer 122 comprises siliconcarbo-nitride (SiCN), while other dielectric materials may be used. Etchstop layer 122 may have a thickness in the range between about 3 nm andabout 10 nm.

Referring to FIG. 28, etch stop layer 122 is formed as a conformallayer, and hence covers the top surfaces and the sidewalls ofsemiconductor fins 124 and silicon germanium oxide regions 38. Inaccordance with some embodiments, thickness T5 of etch stop layer 122 isin the range between about 3 nm and about 10 nm.

Next, as shown in FIG. 29, dielectric regions 128 are formed to filltrenches 32 (FIG. 28), for example, using FCVD. Dielectric regions 128may comprise silicon oxide in accordance with some embodiments. The topsurfaces of the remaining dielectric regions 128 are higher than the topsurfaces of silicon germanium oxide regions 38.

FIG. 29 also illustrates the oxidation of the exposed portions of etchstop layer 122, so that the exposed portions of etch stop layer 122 areconverted to dielectric layer 126. When etch stop layer 122 is formed ofSiCN, the resulting dielectric layer comprises silicon oxycarbo-nitride(SiOCN), which has a different etching characteristic than SiCN.Furthermore, SiOCN is easier to be removed using wet etching than SiCN.Accordingly, the conversion makes is possible to remove the exposedportions of etch stop layer 122 without damaging semiconductor fins 124.In accordance with some embodiments of the present disclosure, theoxidation of etch stop layer 122 is performed using furnace anneal (inan oxygen-containing gas), oxygen implantation, or the like.

After dielectric layer 126 is formed, dielectric layer 126 is removed,for example, through wet etch. The resulting structure is shown in FIG.30. As a result, semiconductor fins 124 are exposed. The unconvertedportions of dielectric etch stop layer 122 remain. In thecross-sectional view, the remaining portions of dielectric etch stoplayer 122 have a U-shape (also including L-shapes). In accordance withsome embodiments of the present disclosure, the top surfaces of theremaining etch stop layer 122 are level with or higher than the topsurfaces of silicon germanium oxide regions 38, so that etch stop layer122 also protects silicon germanium oxide regions 38 in the subsequentetching for forming contact openings. In the resulting structure, thevertical portions of etch stop layer 122 may have portions coplanar withsilicon germanium oxide regions 38. Alternatively, the vertical portionsof etch stop layer 122 may be higher than silicon germanium oxideregions 38.

FIG. 31 illustrates a top view showing the formation of dummy gate stack46, which is formed on the top surfaces and the sidewalls ofsemiconductor fins 124. The respective step is shown as step 412 in theprocess flow shown in FIG. 41. The perspective view of dummy gate stack46 may be essentially the same as shown in FIG. 34B. There may not begate spacers formed on the sidewalls of dummy gate stack 46 at thistime. In accordance with some embodiments, dummy gate stack 46 includesdummy gate electrode 48, which may be formed, for example, usingpolysilicon. Dummy gate stack 46 may also include hard mask layer 50,which may include, for example, silicon nitride layer 50A and siliconoxide layer 50B over silicon nitride layer 50A. Dummy gate stack 46 hasa lengthwise direction substantially perpendicular to the lengthwisedirection of semiconductor fins 124, wherein opposite ends ofsemiconductor fins 124 are not covered by dummy gate stack 46.

FIGS. 32 through 38B illustrate the formation of source and drainregions (referred to as source/drain regions hereinafter). The figurenumbers of FIGS. 32 through 38 may be followed by either letter “A” orletter “B,” wherein letter “A” indicates that the respective view isobtained from a plane same as the vertical plane containing line A-A inFIG. 31, and letter “B” indicates that the respective figure (exceptFIG. 34B) is obtained from the plane same as the vertical planecontaining line B-B in FIG. 31. Accordingly, the figures whose numbersare followed by letter “A” show the cross-sectional views ofsource/drain regions, and the figures whose number is followed by letter“B” shows the cross-sectional views of dummy gate stack 46.

FIGS. 32 through 34B illustrate the formation of source/drain templatesfor epitaxially growing source/drain regions. The respective step isshown as step 414 in the process flow shown in FIG. 41. Referring toFIG. 32, dielectric layer 130 is formed, followed by the formation ofdielectric layer 132 over dielectric layer 130. The materials ofdielectric layers 130 and 132 are different from each other. Dielectriclayer 132 may be formed of SiOCN in accordance with some embodiments.Dielectric layer 130 is formed of a material different from the materialof dielectric layer 132. For example, dielectric layer 130 is formed ofsilicon oxide in some embodiments. The formation of dielectric layer 130has the advantageous feature of increasing the widths of the resultingsource/drain regions, as will be discussed in subsequent paragraphs.Dielectric layers 130 and 132 are formed as conformal layers, and hencewill also extend on the sidewalls (as shown in FIG. 34B) and the topsurfaces of dummy gate stack 46.

FIG. 33 illustrates the removal of semiconductor fins 124 throughetching, wherein the removed portions are not covered by dummy gatestack 46 (FIG. 31). The portions of dielectric layers 130 and 132 oversemiconductor fins 124 are also removed in the etching. After theetching of semiconductor fins 124, silicon germanium oxide regions 38(FIG. 32) are also etched. Source/drain recesses 136 are thus formed toextend to portions of substrate 20 that are between STI regions 42.Recesses 136 have substantially vertical sidewalls, which sidewallsinclude the sidewalls of dielectric layer 130 and etch stop layer 122.In accordance with some embodiments, the etching is anisotropic.

Next, an etching step is performed to remove dielectric layer 130, andthe resulting structure is shown in FIGS. 34A and 34B, which illustratea cross-sectional view of the source/drain regions and a perspectiveview of the source/drain areas and the dummy gate stack 46,respectively. The etching may be isotropic using, for example, wetetching. As a result, the lateral widths of recesses 136 are increasedover that in FIG. 33. This may advantageously increase the widths ofsource/drain regions subsequently grown in recesses 136. Furthermore,the bottom surfaces of the remaining portions of dielectric layer 132(referred to as dielectric templates 132 hereinafter) are spaced apartfrom the underlying dielectric regions 128 by gaps 138. Accordingly,dielectric templates 132 are suspended.

As shown in FIG. 34B, which is a perspective view, dielectric templates132 are connected to the portions of dielectric layer 132 on thesidewalls of dummy gate stack 46, and hence will not fall off. Also, theportions of dielectric layer 130 on the sidewalls of dummy gate stack 46may remain, and are exposed to recesses 136.

In a subsequent step, source/drain regions are epitaxially grown inrecesses 136 as shown in FIGS. 34A and 34B. The respective step is shownas step 416 in the process flow shown in FIG. 41. With the existence ofgaps 138 (FIG. 34B), it is easy for precursors to reach the bottoms andthe inner parts of recesses 136, and hence it is less likely theresulting source/drain regions will have voids. FIG. 35 illustrates theresulting source/drain region 58. The materials and the formationprocess of source/drain region 58 are similar to what are shown in FIG.10, and hence are not repeated herein. Source/drain regions 58 includeportions 58A having vertical sidewalls, portions 58B having facets 58′and 58″, portions 58C between etch stop layers 122, and portions 58Dformed in gaps 138 (FIG. 34A).

FIGS. 36A through 37B illustrate the trimming of source/drain regions58, so that the facets 58′ and 58″ in FIG. 35 are removed to formvertical source/drain regions 58. The respective step is shown as step418 in the process flow shown in FIG. 41. Referring to FIG. 36A,dielectric layer 140 is formed. In accordance with some embodiments,dielectric layer 140 is formed of a same material as that of dielectriclayer 132, which may include, for example, SiOCN. As shown in FIG. 36B,which shows dummy gate stack 46, dielectric layer 140 is also formed ondummy gate stack 46 and contacting dielectric layer 132.

Next, as shown in FIGS. 37A and 37B, a dry etch is performed to etch theportions of dielectric layer 140 overlapping source/drain regions 58, sothat source/drain regions 58 are exposed. A trimming step is thenperformed, for example, using anisotropic (dry) etching, and the facetsof source/drain regions 58 are removed. The resulting structures areshown in FIGS. 38A and 38B, which illustrate the source/drain portionsand the dummy gate stack, respectively. As a result of the source/draintrimming, the resulting source/drain regions 58 have substantiallyvertical sidewalls, with no substantial facets remaining. The sidewallsof the exposed source/drain regions 58 are substantially vertical andstraight. Next, a dry etching is performed to remove the portions ofdielectric layers 132 and 140 on the sidewalls of source/drain regions58. Etch stop layer 122 is hence exposed. In the meanwhile, the topsurface of dummy gate stack 46 is also exposed, as shown in FIG. 38B.The remaining portions of dielectric layers 132 and 140 form gatespacers 132/140. It is appreciated that dielectric layers 132 and 140may have distinguishable interfaces since they are formed in differentprocess steps, regardless of whether they are formed of the same ordifferent materials. The formation of dielectric layer 140advantageously increases the thickness of gate spacers, so that in thestructure in FIG. 38B, the top ends of gate spacers 132/140 are higherthan the top surface of polysilicon layer 48. In the resultingstructure, the thickness of gate spacers 132/140 may be in the rangebetween about 3 nm and about 10 nm.

Next, as shown in FIGS. 39A and 39B, ILD 60 is formed. The respectivestep is shown as step 420 in the process flow shown in FIG. 41. A CMPmay then be performed to level the top surfaces of ILD 60, top surfaceof dummy gate stack 46, and gate spacers 132/140 with each other. Insubsequent steps, dummy gate stack 46 (FIG. 39B) is removed, and a gatedielectric (not shown) and gate electrode 72 are formed as a replacementgate, as shown in FIG. 40A. The respective step is shown as step 422 inthe process flow shown in FIG. 41. In the embodiments in whichsemiconductor fins 124 (FIG. 34B) are formed of a homogeneous material,the formation of the replacement gate includes forming an interfacialdielectric layer and a high-k dielectric layer on the sidewalls and thetop surfaces of semiconductor fins 124 (FIG. 34B), forming a conductivematerial over the high-k dielectric layer, and performing a CMP to levelthe top surfaces of the interfacial dielectric layer, the high-kdielectric layer, and the conductive material with the top surface ofILD 60. In alternative embodiments wherein semiconductor fins 124 havethe same structure as semiconductor stack 24 as shown in FIG. 2, thesteps shown in FIGS. 11 through 16B may be performed to form thereplacement gate.

Referring again to FIG. 40A, after the formation of the replacementgate, ILD 60 is etched to form contact opening (occupied by contactplugs 142 as in FIGS. 40A and 40B), wherein source/drain regions 58 areexposed to the contact openings. In the etching of ILD 60, etch stoplayer 122 acts as the etch stop layer for protecting the underlying STIregions 42. The top ends of etch stop layer 122 may be higher than thetop ends of silicon germanium oxide regions 38 by height difference ΔH,which may be in the range between about 2 nm and about 5 nm, so thatsilicon germanium oxide regions 38 are adequately protected from theetching either. In accordance with some embodiments of the presentdisclosure, as shown in FIG. 40A, a majority of etch stop layer 122 ishigher than silicon germanium oxide regions 38. In alternativeembodiments, as shown in FIG. 30, etch stop layer 122 and silicongermanium oxide regions 38 have most portions level with each other.

Next, a silicidation process is performed to form source/drain silicideregions 144 on the sidewalls of source/drain regions 58, followed byfilling the remaining contact openings with a conductive material toform source/drain contact plugs 146. The respective steps are shown assteps 424 and 426 in the process flow shown in FIG. 41. In accordancewith some embodiments of the present disclosure, the silicide regionscomprise nickel silicide, titanium silicide, cobalt silicide, or thelike. Contact plugs 146 may include cobalt, tungsten, or the like.FinFET 74 is thus formed, as shown in FIG. 40A.

FIGS. 40B and 40C illustrate the cross-sectional views of thesource/drain portions of FinFET 74 in accordance with variousembodiments, wherein the cross-sectional views are obtained from theplane A-A in FIG. 40A. In FIG. 40B, after the silicidation process, theremaining metal used for forming metal silicide is removed, and hencecontact plugs 146 are in contact with silicide regions 144. In FIG. 40C,the remaining metal 148 used for forming metal silicide is not removed,with metal 148 including nickel, titanium, cobalt, or the like.

The embodiments of the present disclosure have some advantageousfeatures. As shown in FIG. 40B, neighboring STI regions 42 have distanceW1, which is the width of the strip portion of substrate 20 between STIregions 42. Source/drain regions 58 have lower portions 58C with widthW1. Source/drain regions 58 further have upper portions 58A/58B withwidth W2, which is greater than width W1. For example, width W1 may bein the range between about 2 nm and about 6 nm, and width W2 may be inthe range between about 6 nm and about 12 nm. The width difference(W2−W1) is caused by the formation and the removal of dielectric layer130 (FIG. 32). Accordingly, the width of source/drain regions isadvantageously greater than the width of the underlying substrateportion. In addition, by forming dielectric templates, formingsource/drain regions from the templates, and then trimming thesource/drain regions, the resulting source/drain regions may have agreat height while still have vertical sidewalls. Therefore, silicideregions may be formed on tall and vertical sidewalls of the source/drainregions, and hence the source/drain contact resistance is reduced,resulting in increased saturation currents for the resulting FinFET.

In accordance with some embodiments of the present disclosure, a deviceincludes a first semiconductor strip, a first gate dielectric encirclingthe first semiconductor strip, a second semiconductor strip overlappingthe first semiconductor strip, and a second gate dielectric encirclingthe second semiconductor strip. The first gate dielectric contacts thefirst gate dielectric. A gate electrode has a portion over the secondsemiconductor strip, and additional portions on opposite sides of thefirst and the second semiconductor strips and the first and the secondgate dielectrics.

In accordance with alternative embodiments of the present disclosure, adevice includes a substrate, a first and a second STI region extendinginto the substrate, a silicon germanium oxide layer between the firstand the second STI regions, and a plurality of semiconductor stripsstacked to overlap the silicon germanium oxide layer. A gate dielectricencircles each of the plurality of semiconductor strips, with someportions of the gate dielectric encircling the plurality ofsemiconductor strips being joined together to form a continuous region.A gate electrode is on the gate dielectric. Source and drain regionsconnect to opposite ends of the plurality of semiconductor strips.

In accordance with yet alternative embodiments of the presentdisclosure, a method includes forming a semiconductor stack, whichincludes a first plurality of semiconductor layers and a secondplurality of semiconductor layers laid out with an alternating layout.The semiconductor stack is patterned to form a stack of semiconductorstrips. The second plurality of semiconductor layers in the stack ofsemiconductor strips is removed, with the first plurality ofsemiconductor layers in the stack of semiconductor strip remaining assemiconductor strips. The semiconductor strips are oxidized to formdielectric rings encircling remaining portions of the semiconductorstrips. Gate dielectrics are formed on the semiconductor strips, whereinthe gate dielectrics formed on neighboring ones of the semiconductorstrips are in contact with each other.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a semiconductorstack comprising a first plurality of semiconductor layers and a secondplurality of semiconductor layers laid out alternatingly; patterning thesemiconductor stack to form a stack of semiconductor strips; removingthe second plurality of semiconductor layers in the stack ofsemiconductor strips, with the first plurality of semiconductor layersin the stack of semiconductor strip remaining as semiconductor strips,wherein the removing the second plurality of semiconductor layerscomprises: oxidizing the second plurality of semiconductor layerspartially to form oxide regions; etching the oxide regions; and afterthe oxide regions are etched, etching remaining portions of the secondplurality of semiconductor layers; and forming gate dielectrics on thesemiconductor strips, with each of the gate dielectrics encircling onesof the semiconductor strips.
 2. The method of claim 1 furthercomprising: removing portions of the stack of semiconductor strips toform a source recess and a drain recess, with an intermediate portion ofthe stack of semiconductor strips remaining between the source recessand the drain recess; and epitaxially growing source and drain regionsfrom the source recess and the drain recess.
 3. The method of claim 1,wherein the first plurality of semiconductor layers comprise siliconlayers free from germanium, and the second plurality of semiconductorlayers comprise silicon germanium.
 4. The method of claim 1 furthercomprising: before the forming the semiconductor stack, forming asilicon germanium layer, with the semiconductor stack overlapping thesilicon germanium layer, wherein when the semiconductor stack ispatterned to form the stack of semiconductor strips, the silicongermanium layer is patterned to form a silicon germanium strip; andfully oxidizing the silicon germanium strip when the oxidizing thesecond plurality of semiconductor layers is performed.
 5. The method ofclaim 1 further comprising, before the forming the semiconductor stack,performing an anti-punch-through implantation on a top portion of asemiconductor substrate, with the semiconductor stack formed over thetop portion of the semiconductor substrate.
 6. A method comprising:forming a semiconductor strip stack; removing a portion of thesemiconductor strip stack to form: a first semiconductor strip; and asecond semiconductor strip overlapping the first semiconductor strip,wherein the first semiconductor strip and the second semiconductor stripare spaced apart by a space; forming a silicon germanium oxide regionoverlapped by the first semiconductor strip; forming isolation regionshaving opposing portions on opposite sides of the silicon germaniumoxide region, wherein the silicon germanium oxide region is embedded inthe isolation regions; forming gate dielectrics surrounding the firstsemiconductor strip and the second semiconductor strip, wherein aportion of the gate dielectrics directly underlying the firstsemiconductor strip has a top surface contacting the first semiconductorstrip, and a bottom surface contacting the silicon germanium oxideregion; and forming a source region and a drain region, wherein thesource region and the drain region are electrically coupled to eachother by the first semiconductor strip and the second semiconductorstrip.
 7. The method of claim 6, wherein the removing the portion of thesemiconductor strip stack comprises removing a third semiconductor stripbetween the first semiconductor strip and the second semiconductorstrip.
 8. The method of claim 7, wherein the removing the thirdsemiconductor strip comprises: oxidizing the third semiconductor strippartially to form oxide regions on opposite sides of a remaining portionof the third semiconductor strip; and etching the oxide regions.
 9. Themethod of claim 8, wherein, after the oxide regions are etched, theremaining portion of the third semiconductor strip is exposed, and theremoving the third semiconductor strip further comprises etching theremaining portion of the third semiconductor strip.
 10. The method ofclaim 6 further comprising: forming the first semiconductor strip andthe second semiconductor strip as parts of a semiconductor stack, withthe semiconductor stack being formed over a semiconductor substrate;etching the semiconductor stack and the semiconductor substrate to formtrenches; filling the trenches with a dielectric material to form theisolation regions; and recessing the isolation regions to revealsidewalls of the first semiconductor strip and the second semiconductorstrip.
 11. The method of claim 6, wherein the portion of the gatedielectrics having the top surface and the bottom surface is verticallyaligned to a middle portion of the first semiconductor strip, and adielectric material of the portion of the gate dielectrics continuouslyextends from the top surface to the bottom surface.
 12. A methodcomprising: depositing a silicon germanium layer over a semiconductorsubstrate; depositing a semiconductor stack over the silicon germaniumlayer, wherein the semiconductor stack comprises: a first plurality ofsemiconductor layers formed of a first semiconductor material; and asecond plurality of semiconductor layers formed of a secondsemiconductor material different from the first semiconductor material,wherein the first plurality of semiconductor layers and the secondplurality of semiconductor layers are stacked alternatingly; etching thesilicon germanium layer, the semiconductor stack and the semiconductorsubstrate to form a strip stack, wherein remaining portions of the firstand the second semiconductor layers form first semiconductor strips andsecond semiconductor strips, respectively, in the strip stack, and aremaining portion of the silicon germanium layer forms a silicongermanium strip; filling a dielectric material into trenches that aregenerated by the etching to form isolation regions; recessing theisolation regions to reveal the first semiconductor strips and thesecond semiconductor strips; performing an oxidation process to fullyoxidize the silicon germanium strip to generate a first silicongermanium oxide region, and in the oxidation process, the firstsemiconductor strips is partially oxidized to form second silicongermanium oxide regions; and removing the first semiconductor strips.13. The method of claim 12 further comprising: forming a gate dielectricsurrounding each of the second semiconductor strips; and forming a gateelectrode on the gate dielectric.
 14. The method of claim 12, whereinthe first semiconductor strips have a higher percentage of germaniumthan the second semiconductor strips.
 15. The method of claim 12,wherein the removing the first semiconductor strips comprises removingthe second silicon germanium oxide regions through an etching process,and simultaneously removing a top portion of the first silicon germaniumoxide region, wherein a bottom portion of the first silicon germaniumoxide region remains after second silicon germanium oxide regions areremoved.
 16. The method of claim 15 further comprising: forming a sourceregion and a drain region connected to opposite sides of the first andthe second semiconductor strips.
 17. The method of claim 15, whereinafter the etching process, the bottom portion of the first silicongermanium oxide region is at a same level as, and is between, opposingportions of, the isolation regions.
 18. The method of claim 4, whereinthe fully oxidizing the silicon germanium strip forms a silicongermanium oxide region, wherein when the remaining portions of thesecond plurality of semiconductor layers are etched, a top portion ofthe silicon germanium oxide region is etched.
 19. The method of claim18, wherein after the remaining portions of the second plurality ofsemiconductor layers are etched, a bottom portion of the silicongermanium oxide region remains.
 20. The method of claim 12, wherein bothof the silicon germanium layer and the first plurality of semiconductorlayers are formed of silicon germanium, and the silicon germanium layerhas a higher germanium concentration than the first plurality ofsemiconductor layers.